1. Field of the Invention
The present invention relates to communication systems, and more particularly to methods and apparatuses for minimizing signal interference.
2. Description of the Related Art
FIGS. 1A and 1B illustrate a typical satellite based broadcast systems of the related art.
FIG. 1A shows a communications system, specifically a television broadcasting system 20, which transmits and receives audio, video, and data signals via satellite. Although the present invention is described in the context of a satellite-based television broadcasting system, the techniques described herein are equally applicable to other methods of program content delivery, such as terrestrial over-the-air systems, cable-based systems, and the Internet. Further, while the present invention will be described primarily with respect to television content (i.e. audio and video content), the present invention can be practiced with a wide variety of program content material, including video content, audio content, audio and video related content (e.g., television viewer channels), or data content.
Television broadcasting system 20 includes transmission station 26, uplink dish 30, at least one satellite 32, and receiver stations 34A-34C (collectively referred to as receiver stations 34). Transmission station 26 includes a plurality of inputs 22 for receiving various signals, such as analog television signals, digital television signals, video tape signals, original programming signals and computer generated signals containing HTML content. Additionally, inputs 22 receive signals from digital video servers having hard discs or other digital storage media. Transmission station 26 also includes a plurality of timing inputs 24, which provide electronic schedule information about the timing and content of various television channels, such as that found in television schedules contained in newspapers and television guides. Transmission station 26 converts the data from timing inputs 24 into program guide data. Program guide data may also be manually entered at the site of transmission station 26. The program guide data consists of a plurality of “objects”. The program guide data objects include data for constructing an electronic program guide that is ultimately displayed on a user's television.
Transmission station 26 receives and processes the various input signals received on inputs 22 and timing inputs 24, converts the received signals into a standard form, combines the standard signals into a single output data stream 28, and continuously sends output data stream 28 to uplink dish 30. Output data stream 28 is a digital data stream that is typically compressed using MPEG2 encoding, although other compression schemes may be used.
The digital data in output data stream 28 are divided into a plurality of packets, with each such packet marked with a service channel identification (SCID) number. The SCIDs are later used by receiver 64 (shown in FIG. 1B) to identify the packets that correspond to each television channel. Error correction data is also included in output data stream 28.
Output data stream 28 is a multiplexed signal that is modulated by transmission station 26 using standard frequency and polarization modulation techniques. Output data stream 28 preferably includes 16 frequency bands, with each frequency band being either left polarized or right polarized. Alternatively, vertical and horizontal polarizations may be used.
Uplink dish 30 continuously receives output data stream 28 from transmission station 26, amplifies the received signal and transmits the signal 31 to at least one satellite 32. Although a single uplink dish and satellite are shown in FIG. 1, multiple dishes and satellites are preferably used to provide additional bandwidth, and to help ensure continuous delivery of signals.
Satellites 32 revolve in geosynchronous orbit about the earth. Satellites 32 each include a plurality of transponders that receive signals 31 transmitted by uplink dish 30, amplify the received signals 31, frequency shift the received signals 31 to lower frequency bands, and then transmit the amplified, frequency shifted signals 33 back to receiver stations 34.
Receiver stations 34 receive and process the signals 33 transmitted by satellites 32. Receiver stations 34 are described in further detail below with respect to FIG. 1B.
FIG. 1B is a block diagram of one of receiver stations 34, which receives and decodes audio, video and data signals. Typically, receiver station 34 is a “set top box,” also known as an Integrated Receiver Decoder (IRD), which is usually resident in a home or multi-dwelling unit, for reception of satellite broadcasted television signals. Receiver dish 60 can be an Outdoor Unit (ODU), which is usually a smaller dish antenna mounted on a home or multi-dwelling unit. However, receiver dish 60 can also be a larger ground-mounted antenna dish if desired.
Receiver station 34 includes receiver dish 60, alternate content source 62, receiver 64, monitor 66, recording device 68, remote control 86 and access card 88. Receiver 64 includes tuner 70/demodulator/Forward Error Correction (FEC) decoder 71, digital-to-analog (D/A) converter 72, CPU 74, clock 76, memory 78, logic circuit 80, interface 82, infrared (IR) receiver 84 and access card interface 90. Receiver dish 60 receives signals 33 sent by satellite 32, amplifies the signals 33 and passes the signals 33 on to tuner 70. Tuner 70 and demodulator/FEC decoder 71 operate under control of CPU 74.
The CPU 74 operates under control of an operating system stored in the memory 78 or within an auxiliary memory within the CPU 74. The functions performed by CPU 74 are controlled by one or more control programs or applications stored in memory 78. Operating system and applications are comprised of instructions which, when read and executed by the CPU 74, cause the receiver 64 to perform the functions and steps necessary to implement and/or use the present invention, typically, by accessing and manipulating data stored in the memory 78. Instructions implementing such applications are tangibly embodied in a computer-readable medium, such as the memory 78 or the access card 88. The CPU 74 may also communicate with other devices through interface 82 or the receiver dish 60 to accept commands or instructions to be stored in the memory 78, thereby making a computer program product or article of manufacture according to the invention. As such, the terms “article of manufacture,” “program storage device” and “computer program product” as used herein are intended to encompass any application accessible by the CPU 74 from any computer readable device or media. Memory 78 and access card 88 store a variety of parameters for receiver 64, such as a list of channels receiver 64 is authorized to process and generate displays for; the zip code and area code for the area in which receiver 64 is used; the model name or number of receiver 64; a serial number of receiver 64; a serial number of access card 88; the name, address and phone number of the owner of receiver 64; and the name of the manufacturer of receiver 64.
Access card 88 is removable from receiver 64 (as shown in FIG. 1B). When inserted into receiver 64, access card 88 is coupled to access card interface 90, which communicates via interface 82 to a customer service center (not pictured). Access card 88 receives access authorization information from the customer service center based on a user's particular account information. In addition, access card 88 and the customer service center communicate regarding billing and ordering of services.
Clock 76 provides the current local time to CPU 74. Interface 82 is preferably coupled to a telephone jack 83 at the site of receiver station 34. Interface 82 allows receiver 64 to communicate with transmission station 26 as shown in FIG. 1A via telephone jack 83. Interface 82 may also be used to transfer data to and from a network, such as the Internet.
The signals sent from receiver dish 60 to tuner 70 are a plurality of modulated Radio Frequency (RF) signals. The desired RF signal is then downconverted to baseband by the tuner 70, which also generates in-phase and quadrature (I and Q) signals. These two signals are then passed to the demodulator/FEC Application Specific Integrated Circuit (ASIC) 71. The demodulator 71 ASIC then demodulates the I and Q signals, and the FEC decoder correctly identifies each transmitted symbol. The received symbols for Quaternary Phase Shift Keying (QPSK) or 8PSK signals carry two or three data bits, respectively. The corrected symbols are translated into data bits, which in turn are assembled in to payload data bytes, and ultimately into data packets. The data packets may carry 130 data bytes or 188 bytes (187 data bytes and 1 sync byte).
In addition to the digital satellite signals received by receiver dish 60, other sources of television content are also preferably used. For example, alternate content source 62 provides additional television content to monitor 66. Alternate content source 62 is coupled to tuner 70. Alternate content source 62 can be an antenna for receiving off the air National Television Standards Committee (NTSC) signals, a cable for receiving Advanced Television Standards Committee (ATSC) signals, or other content source. Although only one alternate content source 62 is shown, multiple sources can be used.
Initially, as data enters receiver 64, CPU 74 looks for initialization data which is referred to commonly in the industry as a boot object. A boot object identifies the SCIDs where all other program guide objects can be found. Boot objects are always transmitted with the same SCID, so CPU 74 knows that it must look for packets marked with that SCID. The information from the boot object is used by CPU 74 to identify packets of program guide data and route them to memory 78.
Remote control 86 emits Infrared (IR) signals 85 that are received by infrared receiver 84 in receiver 64. Other types of data entry devices may alternatively be used, by way of example and not limitation, such as an ultra-high frequency (UHF) remote control, a keypad on receiver 64, a remote keyboard and a remote mouse. When a user requests the display of a program guide by pressing the “guide” button on remote control 86, a guide request signal is received by IR receiver 84 and transmitted to logic circuit 80. Logic circuit 80 informs CPU 74 of the guide request. In response to the guide request, CPU 74 causes memory 78 to transfer a program guide digital image to D/A converter 72. D/A converter 72 converts the program guide digital image into a standard analog television signal, which is then transmitted to monitor 66. Monitor 66 then displays the TV video and audio signals. Monitor 66 may alternatively be a digital television, in which case no digital to analog conversion in receiver 64 is necessary.
Users interact with the electronic program guide using remote control 86. Examples of user interactions include selecting a particular channel or requesting additional guide information. When a user selects a channel using remote control 86, IR receiver 84 relays the user's selection to logic circuit 80, which then passes the selection on to memory 78 where it is accessed by CPU 74. CPU 74 performs an MPEG2 decoding step on received audio, video, and other packets from FEC decoder 71 and outputs the audio and video signals for the selected channel to D/A converter 72. D/A converter 72 converts the digital signals to analog signals, and outputs the analog signals to monitor 66.
Such communications systems 20, here by example which is shown a television broadcast system 20, have embraced the demand for high quality transmissions made possible by digital technology. As the packets and other data are transmitted from uplink dish 30 to receiver 64, the symbols and bits in packets intended for other receiver stations 34 are typically transmitted down from satellite 32 to receiver 64 on the same frequency, because the transmit frequency is controlled by the limitations of satellites 32, and the transmit frequencies that are available are controlled by government permission for transmission at specific frequencies within the frequency spectrum.
Further, the data frames are coded in such a manner that they can interfere with each other, and receiver 64 cannot tell which packets of data that receiver 64 is supposed to decode and present on monitor 66. Such interference is called “co-channel” interference, where one channel of data interferes with the reception and demodulation of another channel of data. In practical applications, the co-channel interference may also stem from transmission of other system operators, a satellite 32 operating in an adjacent orbital slot, or other spot transmission beams in a spot beam satellite broadcasting system 20.
As communications systems 20 transmits more data, i.e., more channels of programming on a satellite broadcast system that are viewable on monitor 66, the interference between data packets will increase, and, as such, the quality of the signal reception will be poorer.
To make optimal use of the available spectrum and to deliver a high number of different channels of programming, RF transmissions with the same frequencies may be directed to different geographic areas. However in areas bordering the different service areas, it is possible that a receiving station may detect a wanted transmission, but also other co-frequency transmissions. The unwanted transmissions are interference and may severely degrade the overall performance of the wanted channel receiver.
Traditionally, the negative effects of co-channel interference have been minimized by redesigning the frequency assignments assigned to the various transponders or satellites 32. But this will not alleviate the problem beyond a certain point. From the foregoing, it is apparent that there is a need to minimize interference beyond which, interference should be minimized. The minimization of such interference is made easier if the interfering signal can be identified from the background noise. There is therefore a need for a method and apparatus for identifying even weak interfering signal.